This invention relates to holographic display elements used for authentication.
In order to discourage fraudulent manufacture of information cards, and particularly credit cards, card manufacturers have been incorporating a holographic pattern or display on the face of the cards. A hologram is a recording of an interference pattern between two beams of light, usually referred to as the signal beam and the reference beam. The signal beam usually includes the image to be recorded, and the reference beam is a beam of light, usually of constant distribution, i.e., plane or spherical wavefront. Typically, another reference beam which is similar, but not necessarily identical, to the one used to record the hologram, is used to reconstruct the signal beam, which then produces an identifiable image. For most holograms, one cannot identify the recorded image without reconstruction.
The holograms typically used on credit cards are what are known as rainbow holograms. They are produced by placing the image to be recorded near the surface of the holographic substrate during recording. These holograms are visible in ordinary, white light and are familiar to many holders of credit cards. The rainbow hologram can be mass produced on sheets of metallized embossing substrate, and stamped onto a credit card. Such holograms are a deterrent to counterfeiting because their production requires an expensive manufacturing investment. Counterfeiting is still a potential problem, however, because with a large enough investment in equipment counterfeit holograms can be produced. Moreover, detecting a counterfeit hologram may be difficult, because of the inherent difficulty of accurately comparing two holograms with just the naked eye.